8 Chapter 1 Introduction 1.1 The Idea behind General Relativity There was no need for general relativity when Einstein started working on it. There was no experimental data signalling any failure of the Newtonian theory of gravity, except perhaps for the minute advance of the perihelion of Mercury s orbit by 47 per century, which researchers at the time tried to explain by perturbations not included yet into the calculations of celestial mechanics in the Solar System. Einstein found general relativity essentially because he was deeply dissatisfied with some of the concepts of the Newtonian theory, in particular the concept of an inertial system, for which no experimental demonstration could be given. After special relativity, he was convinced quite quickly that trying to build a relativistic theory of gravitation led to conclusions which were in conflict with experiments. Action at a distance is impossible in special relativity because the absolute meaning of space and time had to be given up. The most straightforward way to combine special relativity with Newtonian gravity seemed to start from Poisson s equation for the gravitational potential and to add time derivatives to it so as to make it relativistically invariant. However, it was then unclear how the law of motion should be modified because, according to special relativity, energy and mass are equivalent and thus the mass of a body should depend on its position in a gravitational field. This led Einstein to a result which raised his suspicion. In Newtonian theory, the vertical acceleration of a body in a vertical gravitational field is independent of its horizontal motion. In a special- 1

9 CHAPTER 1. INTRODUCTION 2 relativistic extension of Newton s theory, this would no longer be the case: the vertical gravitational acceleration would depend on the kinetic energy of a body, and thus not be independent of its horizontal motion. This was in striking conflict with experiment, which says that all bodies experience the same gravitational acceleration. At this point, the equivalence of inertial and gravitational mass struck Einstein as a law of deep significance. It became the heuristic guiding principle in the construction of general relativity. This line of thought leads to the fundamental concept of general relativity. It says that it must be possible to introduce local, freely-falling frames of reference in which gravity is locally transformed away. The directions of motion of different freelyfalling reference frames will generally not be parallel: Einstein elevators released at the same height above the Earth s surface but over different locations will fall towards the Earth s centre and thus approach each other. This leads to the idea that space-time is a four-dimensional manifold instead of the rigid, four-dimensional Euclidean space. As will be explained in the following two chapters, manifolds can locally be mapped onto Euclidean space. In a freely-falling reference frame, special relativity must hold, which implies that the Minkowskian metric of special relativity must locally be valid. The same operation must be possible in all freely-falling reference frames individually, but not globally, as is illustrated by the example of the Einstein elevators falling towards the Earth. Thus, general relativity considers the metric of the space-time manifold as a dynamical field. The necessity to match it with the Minkowski metric in freely-falling reference frames means that the signature of the metric must be (, +, +, +) or (+,,, ). A manifold with a metric which is not positive definite is called pseudo-riemannian, or Lorentzian if the metric has the signature of the Minkowski metric. The lecture starts with an introductory chapter describing the fundamental characteristics of gravity, their immediate consequences and the failure of a specially-relativistic theory of gravity. It then introduces in two chapters the mathematical apparatus necessary for general relativity, which are the basics of differential geometry, i.e. the geometry on curved manifolds. After this necessary mathematical digression, we shall return to physics when we introduce Einstein s field equations in chapter 4.

10 CHAPTER 1. INTRODUCTION Fundamental Properties of Gravity Scales The first remarkable property of gravity is its weakness. It is by far the weakest of the four known fundamental interactions. To see this, compare the gravitational and electrostatic forces acting between two protons at a distance r. We have ( ) gravity electrostatic force = Gm2 p e 2 1 = Gm2 p = ! r 2 r 2 e 2 (1.1) This leads to an interesting comparison of scales. In quantum physics, a particle of mass m can be assigned the Compton wavelength λ = mc, (1.2) where Planck s constant h is replaced by merely for conventional reasons. We ask what the mass of the particle must be such that its gravitational potential energy equals its rest mass mc 2, and set Gm 2! = mc 2. (1.3) λ The result is the Planck mass, c m = M Pl = G = GeV g = , (1.4) c 2 which, inserted into (1.2), yields the Planck length and the Planck time λ Pl = G c 3 = cm (1.5) t Pl = λ Pl c = G c 5 = s. (1.6) As Max Planck noted already in , these are the only scales for mass, length and time that can be assigned an objective meaning. The Planck mass is huge in comparison to the mass scales of elementary particle physics. The Planck length and time are commonly interpreted as the scales where our classical description of space-time is expected to break down and must be replaced by an unknown theory combining relativity and quantum physics. 1 Über irreversible Strahlungsvorgänge, Annalen der Physik 306 (1900) 69

11 CHAPTER 1. INTRODUCTION 4 Using the Planck mass, the ratio from (1.1) can be written as Gm 2 p e 2 = 1 α m 2 p M 2 Pl, (1.7) where α = e 2 / c 1/137 is the fine-structure constant. This suggests that gravity will dominate all other interactions once the mass of an object is sufficiently large. A mass scale important for the astrophysics of stars is set by the ratio M Pl M 2 Pl m 2 p = M Pl = g, (1.8) which is almost two solar masses. We shall see at the end of this lecture that stellar cores of this mass cannot be stabilised against gravitational collapse The Equivalence Principle The observation that inertial and gravitational mass cannot be experimentally distinguished is a highly remarkable finding. It is by no means obvious that the ratio between any force exerted on a body and its consequential acceleration should have anything to do with the ratio between the gravitational force and the body s acceleration in a gravitational field. The experimentally well-established fact that inertial and gravitational mass are the same at least within our measurement accuracy was raised to a guiding principle by Einstein, the principle of equivalence, which can be formulated in several different ways. The weaker and less precise statement is that the motion of a test body in a gravitational field is independent of its mass and composition, which can be cast into the more precise form that in an arbitrary gravitational field, no local non-gravitational experiment can distinguish a freely falling, non-rotating system from a uniformly moving system in absence of the gravitational field. The latter is Einstein s Equivalence Principle, which is the heuristic guiding principle for the construction of general relativity. It is important to note the following remarkable conceptual advance: Newtonian mechanics starts from Newton s axioms, which introduce the concept of an inertial reference frame, saying that force-free bodies in inertial systems remain at rest or move

12 CHAPTER 1. INTRODUCTION 5 at constant velocity, and that bodies in inertial systems experience an acceleration which is given by the force acting on them, divided by their mass. Firstly, inertial systems are a deeply unsatisfactory concept because they cannot be realised in any strict sense. Approximations to inertial systems are possible, but the degree to which a reference frame will approximate an inertial system will depend on the precise circumstances of the experiment or the observation made. Secondly, Newton s second axiom is, strictly speaking, circular in the sense that it defines forces if one is willing to accept inertial systems, while it defines inertial systems if one is willing to accept the relation between force and acceleration. A satisfactory, non-circular definition of force is not given in Newton s theory. The existence of inertial frames is postulated. Special relativity replaces the rigid Newtonian concept of absolute space and time by a space-time which carries the peculiar light-cone structure demanded by the universal value of the light speed. Newtonian space-time can be considered as the Cartesian product R R 3. An instant t R in time uniquely identifies the three-dimensional Euclidean space of all simultaneous events. Of course, it remains possible in special relativity to define simultaneous events, but the three-dimensional hypersurface from four-dimensional Euclidean space R 4 identified in this way depends on the motion of the observer relative to another observer. Independent of their relative motion, however, is the light-cone structure of Minkowskian space-time. The future light cone encloses events in the future of a point p in space-time which can be reached by material particles, and its boundary is defined by events which can be reached from p by light signals. The past light cone encloses events in the past of p from which material particles can reach p, and its boundary is defined by events from which light signals can reach p. Yet, special relativity makes use of the concept of inertial reference frames. Physical laws are required to be invariant under transformations from the Poincaré group, which translate from one inertial system to another. General relativity keeps the light-cone structure of special relativity, even though its rigidity is given up: the orientation of the light cones can vary across the space-time. Thus, the relativity of distances in space and time remains within the theory. However, it is one of the great achievements of general relativity that it finally replaces the concept of inertial systems by something else which can be experimentally demonstrated: the principle of equivalence

13 CHAPTER 1. INTRODUCTION 6 replaces inertial systems by non-rotating, freely-falling frames of reference. 1.3 Immediate Consequences of the Equivalence Principle Without any specific form of the theory, the equivalence principle immediately allows us to draw conclusions on some of the consequences any theory must have which is built upon it. We discuss two here to illustrate its general power, namely the gravitational redshift and gravitational light deflection Gravitational Redshift We enter an Einstein elevator which is at rest in a gravitational field at t = 0. The elevator is assumed to be small enough for the gravitational field to be considered as homogeneous within it, and the (local) gravitational acceleration be g. According to the equivalence principle, the downward gravitational acceleration felt in the elevator cannot locally be distinguished from a constant upward acceleration of the elevator with the same acceleration g. Adopting the equivalence principle, we thus assume that the gravitational field is absent and that the elevator is constantly accelerated upward instead. At t = 0, a photon is emitted by a light source at the bottom of the elevator, and received some time t later by a detector at the ceiling. The time interval t is determined by h + g 2 t2 = c t, (1.9) where h is the height of elevator. This equation has the solution t ± = 1 [ c ± c2 2gh ] = c g g 1 1 2gh c h 2 c ; (1.10) the other branch makes no physical sense. When the photon is received at the ceiling, the ceiling moves with the velocity v = g t = gh (1.11) c compared to the floor when the photon was emitted. The photon is thus Doppler shifted with respect to its emission, and is received

14 CHAPTER 1. INTRODUCTION 7 with the longer wavelength ( λ 1 + v ) ( λ = 1 + gh ) λ. (1.12) c c 2 The gravitational acceleration is given by the gravitational potential Φ through g = Φ gh Φ, (1.13) where Φ Φ h is the change in Φ from the floor to the ceiling of the elevator. Thus, the equivalence principle demands a gravitational redshift of z λ λ λ = Φ c 2. (1.14) Gravitational Light Deflection Similarly, it can be concluded from the equivalence principle that light rays should be curved in gravitational fields. To see this, consider again the Einstein elevator from above which is at rest in a gravitational field g = Φ at t = 0. As before, the equivalence principle asserts that we can consider the elevator as being accelerated upwards with the acceleration g. Suppose now that a horizontal light ray enters the elevator at t = 0 from the left and leaves it at a time t = w/c to the right, if w is the horizontal width of the elevator. As the light ray leaves the elevator, the elevator s velocity has increased to v = g t = Φ w (1.15) c such that, in the rest frame of the elevator, it leaves at an angle α = v c = Φ w c 2 (1.16) downward from the horizontal because of the aberration due to the finite light speed. Since the upward accelerated elevator corresponds to an elevator at rest in a downward gravitational field, this leads to the expectation that light will be deflected towards gravitational fields. Although it is possible to construct theories of gravity which obey the equivalence principle and do not lead to gravitational light deflection, the bending of light in gravitational fields is by now a well-established experimental fact.

15 CHAPTER 1. INTRODUCTION Impossibility of a Theory of Gravity with Flat Spacetime Gravitational Redshift We had seen before that the equivalence principle implies a gravitational redshift, which has been demonstrated experimentally. We must thus require from a theory of gravity that it does lead to gravitational redshift. Suppose we wish to construct a theory of gravity which retains the Minkowski metric η µν. In such a theory, how ever it may look in detail, the proper time measured by observers moving along a world line x µ (λ) from λ 1 to λ 2 is τ = λ2 λ 1 dλ η µν ẋ µ ẋ ν, (1.17) where the minus sign under the square root appears because we choose the signature of η µν to be ( 1, 1, 1, 1). Now, let a light ray propagate from the floor to the ceiling of the elevator in which we have measured gravitational redshift before. Specifically, let the light source shine between coordinates times t 1 and t 2. The emitted photons will propagate to the receiver at the ceiling along world lines which may be curved, but must be parallel because the metric is constant. The time interval within which the photons arrive at the receiver must thus equal the time interval t 2 t 1 within which they left the emitter. Thus there cannot be gravitational redshift in a theory of gravity in flat spacetime A Scalar Theory of Gravity and the Perihelion Shift Let us now try and construct a scalar theory of gravity starting from the field equation Φ = 4πGT, (1.18) where Φ is the gravitational potential and T = T µ µ is the trace of the energy-momentum tensor. Note that Φ is made dimensionless here by dividing it by c 2. In the limit of weak fields and non-relativistic matter, this reduces to Poisson s equation 2 Φ = 4πGρ, (1.19)

19 CHAPTER 1. INTRODUCTION 12 which allows us to write (1.43) in the form For the Sun, M = g, thus φ = πgm ac 2 (1 ɛ 2 ). (1.45) GM c 2 = cm. (1.46) For Mercury, a = cm and the eccentricity ɛ = 0.2 can be neglected because it appears quadratic in (1.45). Thus, we find φ = radian = (1.47) per orbit. Mercury s orbital time is 88 d, i.e. it completes about 415 orbits per century, so that the perihelion shift predicted by the scalar theory of gravity is per century. φ 100 = 7 (1.48) This turns out to be wrong: Mercury s perihelion shift is six times as large, and not even the sign is right. Therefore, our scalar theory of gravity fails in its first comparison with observations.

20 Chapter 2 Differential Geometry I 2.1 Differentiable Manifolds By the preceding discussion of how a theory of gravity may be constructed which is compatible with special relativity, we are led to the concept of a space-time which looks like Minkowskian space-time locally, but may globally be curved. This concept is cast into a mathematically precise form by the introduction of a manifold. An n-dimensional manifold M is a topological Hausdorff space with a countable base, which is locally homeomorphic to R n. This means that for every point p M, an open neighbourhood U of p exists together with a homeomorphism h which maps U onto an open subset U of R n, h : U U. (2.1) A trivial example for an n-dimensional manifold is the R n itself, on which h may be the identity map id. Thus, h is a specialisation of a map φ from one manifold M to another manifold N, φ : M N. A topological space is a set M together with a collection T of subsets T i M with the properties (i) T and M T; (ii) n i=1 T i T for any finite n; (iii) n i=1 T i T for any n. In a Hausdorff space, any two points x, y M with x y can be surrounded by disjoint neighbourhoods. The homeomorphism h is called a chart or a coordinate system in the language of physics. U is the domain or the coordinate neighbourhood of the chart. The image h(p) of a point p M under the chart h is expressed by the n real numbers (x 1,... x n ), the coordinates of p in the chart h. A set of charts h α is called an atlas of M if the domains of the charts cover all of M. An example for a manifold is the n-sphere S n, for which the twosphere S 2 is a particular specialisation. It cannot be continuously mapped to R 2, but pieces of it can. 13

21 CHAPTER 2. DIFFERENTIAL GEOMETRY I 14 We can embed the two-sphere into R 3 and describe it as the point set S 2 = {(x 1, x 2, x 3 ) R 3 (x 1 ) 2 + (x 2 ) 2 + (x 3 ) 2 = 1} ; (2.2) then, the six half-spheres U ± i defined by U ± i = {(x 1, x 2, x 3 ) S 2 ± x i > 0} (2.3) can be considered as domains of maps which cover S 2 completely, and the charts can be the projections of the half-spheres onto open disks such as D i j = {(x i, x j ) R 2 (x i ) 2 + (x j ) 2 < 1}, (2.4) f + 1 : U+ 1 D 23, f + 1 (x1, x 2, x 3 ) = (x 2, x 3 ). (2.5) Thus, the charts f ± i, i {1, 2, 3}, together form an atlas of the two-sphere. Let now h α and h β be two charts, and U αβ U α U β be the intersection of their domains. Then, the composition of charts h β h 1 α exists and defines a map between two open sets in R n which describes the change of coordinates or a coordinate transform on the intersection of domains U α and U β. An atlas of a manifold is called differentiable if the coordinate changes between all its charts are differentiable. A manifold, combined with a differentiable atlas, is called a differentiable manifold. Using charts, it is possible to define differentiable maps between manifolds. Let M and N be differentiable manifolds of dimension m and n, respectively, and φ : M N be a map from one manifold to the other. Introduce further two charts h : M M R m and k : N N R n whose domains cover a point p M and its image φ(p) N. Then, the combination k φ h 1 is a map from the domain M to the domain N, for which it is clear from advanced calculus what differentiability means. Unless stated otherwise, we shall generally assume that coordinate changes and maps between manifolds are C, i.e. their derivatives of all orders exist and are continuous. We return to the two-sphere S 2 and the atlas of the six projection charts A = { f ± 1, f ± 2, f ± 3 } described above and investigate whether it is differentiable. For doing so, we arbitrarily pick the charts f + 3 and f + 1, whose domains are the northern and eastern half-spheres, respectively, which overlap on the north-eastern

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